François Arago was a French physicist and astronomer who made major contributions to the early study of electromagnetism including the phenomenon of magnetic rotation and the fact that a wire coil could be magnetized by passing electrical current through it. Along with Fresnelon he discovered the principles governing the polarization of light and established the theory of light as a wave. He also studied the velocity of sound, and made it possible to invent the polariscope. Began the studies of the velocity of light which lead to discoveries by Léon Foucault.

Working with Jean-Baptiste Biot, Arago made measurements of arc length on the Earth which led to the standardisation of the metric system of lengths. Arago’s studies in astronomy included investigations of the solar corona and chromosphere, measurements of the diameters of the planets, and a theory that light interference is responsible for the twinkling of stars. Arago also investigated the compressibility, density, diffraction, and dispersion of gases and the various types of lightening.

Arago was also active politically for France's leftist republican cause, serving several political roles in government in addition to his scientific duties (which included Director of the Paris Observatory). In the provisional government, which took power after the 1848 Revolution, he was minister of war and marine. He introduced a number of reforms including the abolition of slavery in the French colonies. He was imprisoned however in 1851 after Napolean III’s coup and he died in 1853 at the age of 70 of complications from dropsy, diabetes, and Bright's Disease.

One of nine children, Arago had a few noteworthy relatives. His brother Etienne, who outlived him, was a hero of the 1848 Revolution and the Commune of 1870, and brother Jean was a hero of the Mexican War of Independence. Arago’s friends included Alexander von Humboldt, Eugène Delacroix, and Prosper Mérimée. Arago had been awarded the Royal Society Copley Medal in 1825 and both Earth's Moon and Mars have craters named in his honor.